The invention relates to a method for automatically correcting frame faults in video assist frames of a video assist system, and to an apparatus for carrying out the method.
U.S. Pat. No. 4,928,171 discloses a video assist system for a motion-picture camera, in which a video image sensor is arranged in an optical beam path of the motion-picture camera, which is interrupted periodically depending on the frame capture frequency of the motion-picture camera. A video assist system of this type serves to generate a video image in parallel with the exposure of film frames of the motion-picture film, which video image on the one hand enables image viewing independently of looking into the camera viewfinder during film capture and on the other hand facilitates the postprocessing of the exposed motion-picture film on the basis of the recorded video sequences.
For this purpose, part of the capture beam path of the motion-picture camera is branched off into a video beam path and directed to the video image sensor of the video assist system or a video camera. In this case, the camera lens of the motion-picture camera projects a frame in the image plane of the motion-picture film, which is moved intermittently for example at a film transport rate of 24 frames per second, if a rotating mirror diaphragm arranged in the capture beam path of the motion-picture camera downstream of the camera lens, with a diaphragm aperture sector, releases the capture beam path during the exposure of a film frame. In the time in which the motion-picture film is transported further by a film frame division, the diaphragm mirror sector of the rotating mirror diaphragm conceals the capture beam path to the image plane and directs the film frame onto the plane of a ground glass screen or fiber plate, from which the image that arises there is imaged on the video image sensor via a video assist lens.
The video image sensor integrates the light of the video beam path that falls onto its light-sensitive layer. The integrated signals are periodically read out from the video image sensor and represented as video output signals on a video monitor or stored on a suitable storage medium.
Situated between the ground glass screen or fiber plate and the video assist lens is at least one further beam splitter which branches off the image of the ground glass screen to an eyepiece via which a cameraman can view the film frame on the ground glass screen. Via a further beam splitter, a possibly illuminated format depiction can be imaged on the ground glass screen, which makes it easier for the cameraman to identify the frame center and the frame limits of the exposed film frame primarily in the dark.
The ground glass screens used for visually assessing an image or motif to be captured and for imaging the video assist frames are frosted either by a grinding process or in a chemical etching process, while fiber plates are produced from a bundle of individual light-guiding fibers that are oriented parallel. Both a ground glass screen and a fiber plate have a structure, however, which is disturbing particularly in the case of small-format frames such as are customary when capturing films. Although these structures can be reduced in the case of a ground glass screen by using a particularly fine abrasive grain during the grinding process, the ground glass screen becomes too transparent as a result, whereby the assessment of the optimum focus setting becomes impossible and the image brightness becomes nonuniform. The same disadvantages occur in the case of ground glass screens which are coated with a transparent lacquer in order to reduce the granularity on their matt surface.
Fiber plates have the advantage of a ground glass screen that the structure is smaller, but they are more expensive than ground glass screens and, particularly at relatively small apertures of the imaging camera lens, reveal a regular, usually honeycomb-shaped, structure that disturbs the artistic assessment of the image or motif. In this case, the structures of the ground glass screen or fiber plate are discernible all the more clearly, the more the camera lens is stopped down.
In order to improve the image quality of an image imaged on a ground glass screen or fiber plate, it is known from DE 100 20 307 to connect the ground glass screen or fiber plate to a drive device which moves the ground glass screen or fiber plate in oscillatory fashion at a frequency lying significantly above the temporal resolution capability of the eyes at approximately 60 hertz. With the aid of such an arrangement, although the appearance of the granularity and the honeycomb-shaped structure can be prevented or at least significantly reduced, the mechanical movement of the ground glass screen or fiber plate can be realized only with considerable outlay within a motion-picture camera and is also unable to solve the problem of optical vignetting explained below.
Before the video beam path branched off from the capture beam path reaches the video image sensor, the beam path passes firstly through the camera lens and then through the video assist lens. If there were no ground glass screen present between the two lenses, then it would be necessary, in order to avoid a keyhole effect, in which a decrease in brightness toward the edge of the video assist frame, that is to say optical vignetting, occurs, to meet the condition stipulating that the exit pupil of the camera lens lies in the entrance pupil of the video assist lens and said entrance pupil is smaller than the exit pupil of the camera lens. This condition cannot be met in practise, however, because there are a large number of camera lenses from the same or different manufacturers and the exit pupils of the respective camera lenses lie at different locations. One major reason for this is that the pupil position of a camera lens is irrelevant to the latter's essential task of imaging a capture object on the motion-picture film.
However, even when a ground glass screen or fiber plate is arranged between the camera lens and the video assist lens, optical vignetting or a brightness decrease toward the edge of the video assist frame occurs because the ground glass screen does not fully compensate for the different pupil positions between the camera lens and the video assist lens. Only an ideal ground glass screen behaving like a Lambertian emitter, whose luminance is constant in all directions and thus forms an ideal diffusely emitting area, would be able to compensate for the different pupil positions between the camera lens and the video assist lens and thus fully eliminate the keyhole effect. In this case, however, light would be sent into all spatial segments and only a fraction would come into the entrance pupil of the video assist or the eyepiece at the optical viewfinder, whereby a very dark image would arise there.
A further component-dictated frame fault in the generation of video assist frames by means of a video assist system is caused by the inherent noise of the video image sensor comprising a semiconductor component and occurs in or at different locations or pixels of the video assist frame in the case of successive video frames.